Tape drive and printing apparatus

ABSTRACT

A tape drive for use in for example a transfer printing apparatus to drive a printer ribbon. The printer ribbon is mounted on two spools each of which is driven by a respective stepper motor. A controller controls the energization of the motor such that the ribbon is transported in at least one direction between spools mounted on the spool support. The controller is operative to energize both motors to drive the spools of ribbon in the direction of ribbon transport to achieve push-pull operations. Ribbon tension is monitored to enable accurate control of ribbon supply and ribbon take-up, the ribbon tension being monitored, for example, by monitoring power supply to the two stepper motors.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/380,182, filed Mar. 16, 2004, which is a US national phase ofinternational application PCT/GB01/03965 which designated the U.S. andwas filed Sep. 5, 2001, and claims benefit of GB 0022206.7 dated Sep.11, 2000; GB 0028465.3 dated Nov. 22, 2000; GB 0100493.6 dated Jan. 9,2001, and GB 0111044.4 dated May 2, 2001, the entire contents of all ofthese applications are hereby incorporated by reference.

BACKGROUND

1. Technical Field

This invention generally relates to tape drive and printing apparatusand to methods of operation for same and to apparatus and methods whichmay be used in transfer printers, that is printers which make use ofcarrier-supported inks.

2. Related Art

In transfer printers, a tape which is normally referred to as a printerribbon and carries ink on one side is presented within a printer suchthat a print head can contact the other side of the ribbon to cause theink to be transferred from the ribbon onto a target substrate of forexample paper or a flexible film. Such printers are used in manyapplications. Industrial printing applications include thermal transferlabel printers and thermal transfer coders which print directly onto asubstrate such as packaging materials manufactured from flexible film orcard.

Ink ribbon is normally delivered to the end user in the form of a rollwound onto a core. The end user pushes the core onto a tape spool, pullsa free end of the roll to release a length of ribbon, and then engagesthe end of the tape with a further spool. Generally the spools aremounted on a cassette which can be readily mounted on a printingmachine. The printing machine includes a transport means for driving thetwo spools, so as to unwind ribbon from one spool and to take up ribbonon the other spool. The printing apparatus transports ribbon between thetwo spools along a predetermined path past the printing head.

Known printers of the above type rely upon a wide range of differentapproaches to the problem of how to drive the ribbon spools. Some relyupon stepper motors,others on DC motors to directly or indirectly drivethe spools. Generally the known arrangements drive only the spool ontowhich ribbon is taken up (the take-up spool) and rely upon some form of“slipping clutch” arrangement on the spool from which ribbon is drawn(the supply spool) to provide a resistive force so as to ensure that theribbon is maintained in tension during the printing and ribbon windingprocesses and to prevent ribbon overrun when the ribbon is brought torest. It will be appreciated that maintaining adequate tension is anessential requirement for proper functioning of the printer.

As a roll of ribbon is gradually used by the printer, the initialoutside diameter of the supply spool decreases and the initial outerdiameter of the take-up spool increases. In slipping clutch arrangementswhich offer an essentially constant resistive torque, the ribbon tensionwill vary in proportion to the diameter of the spools. Given that it isdesirable to use large supply spools so as to minimize the number oftimes that a ribbon roll has to be replenished, this is a seriousproblem particularly in high speed machines where rapid ribbon transportis essential.

Dynamically changing ribbon tension gives rise to demands for tighttolerances for the torque delivered by the slipping clutch. Suchtolerances are difficult to maintain as wear in the slipping clutch overtime tends to change the resistive force exerted by the clutch. If theclutch force is too great the ribbon transport system may haveinadequate power to drive the ribbon throughout the range of spooldiameters from a new supply roll to an empty supply roll. Too littleclutch force and slack in the ribbon could result in overrun of thesupply spool. Given these constraints, typical printer designs havecompromised performance by way of limiting the rate of acceleration, therate of deceleration, and the maximum speed capability of the ribbontransport system. Overall printer performance has as a result beencompromised.

Representative examples of conventional printing apparatus are describedin U.S. Pat. No. 4,000,804, U.S. Pat. No. 4,294,552, U.S. Pat. No.4,479,081, U.S. Pat. No. 4,788,558 and British patent 2310405.

The system of U.S. Pat. No. 4,000,804 describes an arrangement fortransferring a ribbon from a supply spool to a take-up spool whichincludes a pair of electric motors each one of which is connected to acorresponding spool shaft. The motors are direct current (DC) motors.The motor connected to the take-up spool is supplied by a constantcurrent generator so as to wind up the ribbon with a substantiallyconstant torque. The motor connected to the supply spool is supplied bya constant voltage generator so as to keep the ribbon tensioned duringribbon transfer. A change-over device alternates the function of the twospools when the ribbon is fully wound on the take-up spool. With thedescribed arrangement, no account is taken of the change in diameters ofthe supply and take-up spools during ribbon transfer and thus ribbontension varies substantially during the course of the full transfer ofthe ribbon from the supply spool to the take-up spool.

U.S. Pat. No. 4,294,552 discloses a bi-directional ribbon drive in whichtwo spools are driven by respective stepper motors. The take-up spool isdriven by its stepper motor, but the supply spool motor is fed a lowlevel “drag” current to maintain the ribbon in tension. The motors arenot actively controlled to compensate for spool diameter variations.

U.S. Pat. No. 4,479,081 describes an arrangement in which two steppermotors are provided, one driving the take-up spool and the other coupledto the supply spool. Feedback signals provide an indication of theangular velocity of the supply spool and a function table providesinformation on the rate of stepping pulses to be applied to the take-upspool. The ribbon is driven by the stepper motor driving the take-upspool, the other motor acting as a feedback transducer to enableappropriate control of the motor driving the take-up spool to takeaccount of changing spool diameters while maintaining a constant ribbonspeed. Thus although this arrangement does avoid the need for example ofa capstan drive interposed between the two spools so as to achievereliable ribbon delivery speeds, only one of the motors is driven todeliver torque to assist ribbon transport. There is no suggestion thatthe apparatus can operate in push-pull mode, that is the motor drivingthe take-up spool operating to pull the ribbon and the motor driving thesupply spool operating to push the associated spool in a direction whichassists tape transport.

U.S. Pat. No. 4,788,558 describes a ribbon drive mechanism in which twoDC motors are provided, one driving the take-up spool and one drivingthe supply spool. Ribbon is delivered by a further drive roller drivenby a stepper motor. The supply spool DC motor acts as a brake and doesnot assist in tape transport. Thus this is a conventional arrangement inwhich a capstan roller is used to control ribbon delivery speed. Withsuch an arrangement it is a relatively simple matter as described toprovide feedback information concerning the magnitude of the ribbonspools so as to maintain a desired ribbon tension, but the overallsystem is complex.

GB 2310405 describes a bi-directional printer ribbon drive mechanism inwhich a stepper motor drives a take-up spool. Accurate control of ribbondelivery is achieved by providing an idler roller which rotates incontact with the ribbon and thus enables a direct measurement of ribbontransport speed. The provision of such an idler roller and associatedcomponents adds to overall system complexities and cost.

None of the known arrangements is capable of coping well with therequirements of high speed industrial transfer printing systems. Suchsystems generally operate in one of two manners, that is eithercontinuous printing or intermittent printing. In both modes ofoperation, the apparatus performs a regularly repeated series ofprinting cycles, each cycle including a printing phase during which inkis being transferred to a substrate, and a further non-printing phaseduring which the apparatus is prepared for the printing phase of thenext cycle.

In continuous printing, during the printing phase a stationary printhead is brought into contact with a printer ribbon the other side ofwhich is in contact with a substrate onto which an image is to beprinted. (The term “stationary” is used in the context of continuousprinting to indicate that although the print head will be moved into andout of contact with the ribbon, it will not move relative to the ribbonpath in the direction in which ribbon is advanced along that path). Boththe substrate and printer ribbon are transported past the print head,generally but not necessarily at the same speed. Generally onlyrelatively small lengths of the substrate which is transported past theprinter head are to be printed upon and therefore to avoid gross wastageof ribbon it is necessary to reverse the direction of travel of theribbon between printing operations. Thus in a typical printing processin which the substrate is traveling at a constant velocity, the printhead is extended into contact with the ribbon only when the print headis adjacent regions of the substrate to be printed. Immediately beforeextension of the print head, the ribbon must be accelerated up to forexample the speed of travel of the substrate. The ribbon speed must thenbe maintained at the constant speed of the substrate during the printingphase and, after the printing phase has been completed, the ribbon mustbe decelerated and then driven in the reverse direction so that the usedregion of the ribbon is on the upstream side of the print head. As thenext region of the substrate to be printed approaches, the ribbon mustthen be accelerated back up to the normal printing speed and the ribbonmust be positioned so that an unused portion of the ribbon close to thepreviously used region of the ribbon is located between the print headand the substrate when the print head is advanced to the printingposition. Thus very rapid acceleration and deceleration of the ribbon inboth directions is required, and the ribbon drive system must be capableof accurately locating the ribbon so as to avoid a printing operationbeing conducted when a previously used portion of the ribbon isinterposed between the print head and the substrate.

In intermittent printing, a substrate is advanced past a print head in astepwise manner such that during the printing phase of each cycle thesubstrate and generally but not necessarily the ribbon are stationary.Relative movement between the substrate, ribbon and print head areachieved by displacing the print head relative to the substrate andribbon. Between the printing phase of successive cycles, the substrateis advanced so as to present the next region to be printed beneath theprint head and the ribbon is advanced so that an unused section ofribbon is located between the print head and the substrate. Once againrapid and accurate transport of the ribbon is necessary to ensure thatunused ribbon is always located between the substrate and print head ata time that the print head is advanced to conduct a printing operation.

The requirements in terms of ribbon acceleration, deceleration, speedand positional accuracy of high speed transfer printers is such that theknown drive mechanisms have difficulty delivering acceptable performancewith a high degree of reliability. Similar constraints also apply inapplications other than high speed printers. Accordingly it is an objectof the present exemplary embodiment to provide a tape drive which can beused to deliver printer ribbon in a manner which is capable of meetingthe requirements of high speed production lines, although the tape driveof the present invention may of course be used in other applicationswhere similar high performance requirements are demanded.

BRIEF DESCRIPTION

In an exemplary embodiment, there is provided a tape drive comprisingtwo motors at least one of which is a stepper motor, two tape spoolsupports on which spools of tape may be mounted, each spool supportbeing drivable by a respective motor, and a controller for controllingthe energization of the motors such that tape may be transported in atleast one direction between spools mounted on the spool supports,wherein the controller is operative to energize both motors to drive thespools of tape in the direction of tape transport.

A tape drive in accordance with an exemplary embodiment relies upon boththe motors which drive the two tape spools to drive the tape during tapetransport. Thus the two motors operate in push-pull mode. This makes itpossible to achieve very high rates of acceleration and deceleration.Tension in the tape being transported is determined by control of thedrive motors and therefore is not dependent upon any components whichhave to contact the tape between the take-up and supply spools. Thus avery simple overall mechanical assembly can be achieved. Given that bothmotors contribute to tape transport, relatively small and thereforeinexpensive and compact motors can be used.

The actual rotational direction of each spool will depend on the sensein which the tape is wound on each spool. If both spools are wound inthe same sense then both spools will rotate in the same rotationaldirection to transport the tape. If the spools are wound in the oppositesense to one another, then the spools will rotate in opposite rotationaldirections to transport the tape. In any configuration, both spoolsrotate in the direction of tape transport.

Preferably the controller is arranged to control the motors to transporttape in both directions between the spools. The motors may both bestepper motors and the controller may be operative to monitor tension ina tape being transported between spools mounted on the spool support andto control the motors to maintain the monitored tension betweenpredetermined limits. Means are preferably provided to monitor the powersupply to at least one of the motors and to calculate an estimate oftape tension from the monitored power. For example, where two steppermotors are provided, a power supply may deliver power to a stepper motordrive means which supplies current sequentially to windings of thestepper motors, the power being monitored by means for monitoring themagnitude of voltage and/or current supplied to the motors and/or themotor drive means. It will be appreciated that dependent upon the loadapplied to the motors the current and voltage delivered to the motorwindings will both vary, irrespective of the type and nature of themotor drive means used. For this reason it is preferred to provide aregulated power supply which supplies a substantially constant voltageto the stepper motor drive means and to monitor the magnitude of currentsupplied to the stepper motor drive means from the power supply.

Preferably each stepper motor is energized by a respective motor drivecircuit, a respective low resistance resistor being connected in serieswith each motor drive circuit, and voltage signals developed across theseries resistors being monitored to monitor the current supplied to themotors. The voltage signals may be converted to digital signals forsupply to a microcontroller which controls the generation of motorcontrol pulse trains which are applied to the motor drive circuits. Thecurrent may be monitored over a predetermined period of time andpreferably is monitored only during periods in which tape transportspeed is substantially constant. The predetermined period of time maycorrespond to a predetermined length of tape transport.

Calibration data may be recorded for the or each stepper motor, thecalibration data representing power consumption for the stepper motor ateach of a series of step rates under no tape load conditions, and ameasure of tape tension may be calculated by reference to a measure ofmotor step rate, the calibration data related to the step rate, and thepower consumed by the motor.

The outside diameters of the tape spool may be directly monitored andthe tape tension calculated to take into account the monitoreddiameters. The outside diameters may be monitored for each of aplurality of diameters which are mutually inclined to each other so asto enable the detection of spool eccentricity and therefore enable anaccurate calculation of the spool circumference.

A measure of tension t may be calculated from measures of power suppliedto the two motors, measures of the spool radii, calibration factors forthe two motors related to the step rate of the motors. A calibrationscaling factor may also be used to translate the calculated tension intoa more interpretable value. Preferably the controller implements acontrol algorithm to calculate a length of tape to be added to orsubtracted from the tape extending between the spools in order tomaintain the value t between predetermined limits and to control thestepper motors to add or subtract the calculated length of tape to thetape extending between the spools. Alternatively, a measure of thedifference between the current supplied to the two motors may be derivedand stepping of the motors may be controlled dependent upon thedifference measure. It will be appreciated that the difference measurecould simply be the result of subtracting one current from the other orcould relate to the ratio of the two measured currents. Motor speed maybe maintained constant during a period in which the difference measureis within each of a series of tolerance bands defined between upper andlower limits, and the tolerance bands may be adjusted in dependence uponthe ratio of the outside diameters of the spools. The controlling meansmay implement a control algorithm to calculate a length of tape to beadded to or subtracted from the tape extending between the spools inorder to maintain the difference measure between the upper and lowerlimit and to control the stepper motors to add or subtract thecalculated length of tape to the tape extending between the spools.

A value corresponding to tape width may be input and the predeterminedlimit adjusted to take account of that tape width. For example, thecontrol algorithm may comprise gain constants, and the gain constantsmay be adjusted to take account of tape width. The control algorithm mayoperate cyclically such that during one cycle the length of tape to beadded or subtracted is calculated and during a subsequent cycle themotors are controlled to adjust the amount of tape between the spools.This approach is adopted because, as it will be appreciated thatalthough the length of tape between the spools is to a firstapproximation independent of tension, stretching of the tape will meanthat if tape is added to the length of tape extending between the spoolsthis will be taken up by a reduction in stretch until the tensionbecomes zero. It will be further appreciated that for a given tension, anarrower tape will stretch more than a wider tape, therefore a change intension, caused by the addition or subtraction of tape between thespools, will be less for a narrower tape than for a wider tape.

Tension monitoring makes it possible to generate a fault-indicatingoutput if the measure of tension falls below a minimum acceptable limitto indicate for example a tape breakage.

Spool diameters may be monitored using an optical sensing systemincluding at least one light emitter and at least one light detectorarranged such that an optical path is established there between, atransport mechanism supporting at least one part of the optical sensingsystem and drivable so as to cause the optical path to sweep across aspace within which spools to be measured will be located, and acontroller operative to control the transport mechanism, to detectpositions of the transport mechanism in which the output of the detectorchanges to indicate a transition between two conditions in one of whichthe optical path is obstructed by a spool and in the other of which theoptical path is not obstructed by that spool, and to calculate the spooldiameters from the detected positions of the transport mechanism inwhich the detector output changes.

One of the emitter and detector may be mounted on the transportmechanism, the other being fixed in position relative to the spools oftape, or alternatively both the emitter and detector may be mounted onthe transport mechanism, the optical path between the emitter anddetector being established by a mirror located at the side of the spoolsremote from the transport mechanism and arranged to reflect light fromthe emitter back to the detector. Spool diameters may be measured withthe spools in a first position, the spools may then be rotated so thatone spool rotates by for example 30°, the diameters may be measuredagain, and so on. This makes it possible to accurately assess spooleccentricity and outer circumference.

An exemplary embodiment has particular applicability where the transportmechanism is a print head transport mechanism of a transfer ribbonprinter. The ratio of spools in such a machine can be calculated on thebasis of the output of the diameter measuring means. The ratiocalculating means may comprise means enabling a first stepper motordriving a take-up spool and disabling a second stepper motor driving asupply spool such that the second stepper motor acts as a generator,means for generating pulses from the second stepper motor, the pulserate being proportional to motor speed, means for detecting thegenerated pulses to produce a measure of the rotation of the secondstepper motor, means for monitoring stepping of the first stepper motorto produce a measure of the rotation of the first stepper motor, andmeans for comparing the measures of the rotations of the motors tocalculate the ratio of the outside diameters of the spools.

After a number of operating cycles of the tape drive, in which tape istransported between the spools, an updated diameter for at least one ofthe spools may be calculated from a ratio between the spool diameters asinitially monitored, a current ratio between the spool diameters, andthe diameter of at least one spool as initially monitored.

Where the tape drive in accordance with an exemplary embodiment isincorporated in a transfer printer for transferring ink from a printerribbon to a substrate which is transported along a predetermined pathadjacent to the printer, the tape drive acting as a printer ribbon drivemechanism for transporting ribbon between first and second ribbonspools, the printer may further comprise a print head arranged tocontact one side of the ribbon to press an opposite side of the ribboninto contact with a substrate on the predetermined path, a print headdrive mechanism for transporting the print head along a track extendinggenerally parallel to the predetermined substrate transport path and fordisplacing the print head into and out of contact with the ribbon, and acontroller controlling the printer ribbon and print head drivemechanisms, the controller being selectively programmable either tocause the ribbon to be transported relative to the predeterminedsubstrate transport path with the print head stationary and displacedinto contact with the ribbon during printing, or to cause the print headto be transported relative to the ribbon and the predetermined substratetransport path and to be displaced into contact with the ribbon duringprinting.

The drive mechanism may be bidirectional such that ribbon may betransported from the first spool to the second spool and from the secondspool to the first.

Where the print head is mounted on a print head carriage that isdisplaceable along the track, first and second carriages may be providedwhich are interchangeable and shaped such that with one carriage inposition on the track the print head is disposed so as to enableprinting on a substrate traveling in one direction along the substratetransport path and with the other carriage in position on the track theprint head is disposed so as to enable printing on a substrate travelingin the other direction along the substrate path.

The tape drive may be incorporated in a printing apparatus comprising ahousing, a print head mounted on a print head support assembly which isdisplaceable relative to the housing in a direction parallel to a printribbon path along which a ribbon is driven by the tape drive, a firstdrive mechanism for displacing the print head support relative to thehousing, a roller which in use supports a substrate to be printed on theside of the ribbon path remote from the print head, a second drivemechanism for displacing the print head relative to the print headsupport assembly to a printing position in which a portion of the printhead bears against the roller or any substrate or ribbon interposedbetween the print head and roller, and a controller for adjusting thefirst drive mechanism to adjust the angular position of the print headrelative to the rotation axis of the roller.

Preferably the print head is mounted on a print head support assemblywhich is displaceable relative to the housing in a direction parallel toa print ribbon path along which a ribbon is driven by the tape drive, afirst drive mechanism for displacing the print head support relative tothe housing, a peel off roller mounted on the print head supportassembly and displaceable with the print head in the said paralleldirection, and a second drive mechanism for displacing the print headrelative to the print head support assembly and peel off roller betweena ready to print position adjacent the print ribbon path and a printingposition in which the print head would contact a print ribbon on thepath, wherein a cam mechanism is provided which is engaged as a resultof displacement of the print head support assembly to a predeterminedposition and when engaged causes retraction of the print head away fromthe ready to print position to a position spaced from the peel-offroller and the print ribbon path.

The cam mechanism may comprise a plate mounted in the housing anddefining a slot, and a pin extending from a pivotal member mounted onthe print head support assembly, engagement of the pin in the slot as aresult of displacement of the print head support assembly to thepredetermined position causing the pivotal member to rotate from a firstposition in which it supports the print head to a second position inwhich the print head is free to move to the position spaced from thepeel-off roller and the print ribbon path.

The pivotal member may be mounted on a displaceable member mounted onthe print head support assembly, displacement of the displaceable memberfrom a retracted to an extended position when the pivotal member is inthe first position causing the print head to move from the ready toprint position from the printing position.

The printing apparatus may further comprise means for applying the printhead to a ribbon supported in the drive mechanism, the print headcomprising an array of printing elements each of which may beselectively energized to release ink from a portion of ribbon contactedby that element, and a controller for controlling energization of theprinting elements and the advance of the ribbon so as to perform aseries of printing cycles each of which includes a printing phase duringwhich relative movement between the print head and ribbon results in theprint head traversing a predetermined length of ribbon and anon-printing phase during which the ribbon is advanced a predetermineddistance relative to the print head, wherein the controller is arrangedselectively to energize different groups of printing elements duringsuccessive printing cycles, the groups of elements being distributed onthe print head such that different groups contact different portions ofthe ribbon, and the controller is arranged to advance the ribbon suchthat the said predetermined distance of ribbon advance is less than thesaid predetermined length of ribbon, the groups of printing elementsbeing energized such that that ribbon is advanced by at least saidpredetermined length of ribbon in the interval between any two printingphases in which the same group of printing elements are energized. Twogroups of printing elements may be provided such that the distance ofribbon advance may be as little as half the predetermined length ofribbon.

Given the fundamental differences between continuous and intermittentprinting systems as described above, it has been industry practice toprovide printing apparatus which is capable either of use in acontinuous printing application or for use in an intermittent printingapplication but not to provide a printer with the versatility to performboth functions. The fundamental difference between the two types ofprinting apparatus required for these two applications is that in one(continuous printing) the print head is stationary (using that term inthe manner discussed above) whereas in the other (intermittent) theprinting head must be displaceable. As a result, when a particularproduction line is converted from for example an intermittent printingapplication to a continuous printing application it is necessary toreplace all of the printing equipment. This represents a considerablecost to users of such equipment.

It is an object of an exemplary embodiment to obviate or mitigate theproblems outlined above.

According to a second aspect of an exemplary embodiment, there isprovided a transfer printer for transferring ink from a printer ribbonto a substrate which is transported along a predetermined path adjacentthe printer, comprising a printer ribbon drive mechanism fortransporting ribbon between first and second ribbon spools, a print headarranged to contact one side of the ribbon to press an opposite side ofthe ribbon into contact with a substrate on the predetermined path, aprint head drive mechanism for transporting the print head along a trackextending generally parallel to the predetermined substrate transportpath and for displacing the print head into and out of contact with theribbon, and a controller controlling the printer ribbon and print headdrive mechanisms, the controller being selectively programmable eitherto cause the ribbon to be transported relative to the predeterminedsubstrate transport path with the print head stationary and displacedinto contact with the ribbon during printing, or to cause the print headto be transported relative to the ribbon and the predetermined substratetransport path and to be displaced into contact with the ribbon duringprinting.

Thus the second aspect of this exemplary embodiment provides a printingapparatus with sufficient versatility to be able to be used in bothcontinuous and intermittent applications.

The transfer printer of the second aspect of the exemplary embodiment asdefined above may be used in conjunction with any or all of the featuresof the first aspect of the exemplary embodiment as discussed above. Inparticular, the drive mechanism may be bidirectional, the drivemechanism may comprise two stepper motors each driving a respective oneof the first and second ribbon spools in the direction of tapetransport, ribbon tension may be monitored and the stepper motorscontrolled to maintain the monitored tension within predeterminedlimits, the print head drive mechanism may comprise a further steppermotor coupled to the print head, and the print head may be mounted on acarriage that is displaceable along a track. In addition, first andsecond carriages which are interchangeable may be provided to enableprinting on a substrate traveling in either direction along thesubstrate transport path and a peel off roller mounted adjacent theprint head may be reversible in position relative to the print head.

As outlined above, in tape drives which are used to transfer tape suchas a printer ribbon between two spools, the diameters of the spoolschange during the course of tape transfer from one spool to the other.This dramatically affects the relative speeds of the two spools whichmust be maintained if the tape is to remain in tension. Various attemptshave been made to account for this effect, and notably the approachadopted in U.S. Pat. No. 4,479,081. None of the known approaches howeveris satisfactory in delivering a reliable accurate measurement of spooldiameters to thereby enable an accurate and appropriate control of drivemotor speeds in an arrangement in which the two motors are operating inpush-pull mode. Whereas some of the known systems can cope with tapedrives in which the initial conditions are always the same (for examplea fresh supply spool of known outside diameter is connected to an emptytake-up spool), in many applications it is quite often the case that anoperator will fit to a machine a tape which has been partially used suchthat the supply spool which initially was of known outside diameter haspartly been transferred to the take-up spool.

It is an object of an exemplary embodiment to obviate or mitigate theproblems outlined above.

According to a third aspect of an exemplary embodiment, there isprovided an apparatus for measuring the diameters of two spools of tapemounted on a tape drive mechanism which is drivable to transport tapebetween the spools, comprising an optical sensing system including atleast one light emitter and at least one light detector arranged suchthat an optical path is established there between, a transport mechanismsupporting at least part of the optical sensing system and drivable soas to cause the optical path to sweep across a space within which spoolsto be measured will be located, and a controller operative to controlthe transport mechanism, to detect positions of the transport mechanismin which the output of the detector changes to indicate a transitionbetween two conditions in one of which the optical path is obstructed bya spool and in the other of which the optical path is not obstructed bythat spool, and to calculate the spool diameters from the detectedpositions of the transport mechanism in which the detector outputchanges.

This third aspect of an exemplary embodiment makes it possible toaccurately determine spool sizes. In an apparatus such as a transferprinter with a displaceable print head the displaceable component can bereadily mounted on the displaceable print head so as to require noadditional electromechanical components over and above those necessaryfor the normal functioning of the apparatus.

The apparatus of the third aspect of the exemplary embodiment as definedabove may be used in conjunction with any of the features of the firstand second aspects of the exemplary embodiment as defined above.

Print heads used in for example transfer printers must be accuratelypositioned relative to a platen which supports a substrate to be printedif good quality print is to be produced, particularly at high printingspeeds. An angular displacement of only a few degrees can radicallyaffect print quality. The traditional approach to dealing with thisproblem is to position a print head on an appropriate support assemblyin a nominally correct position, to then run a test print to see whatquality results, and to then mechanically adjust the position of theprint head so as to optimize print quality. This involves an installermaking very small mechanical adjustments using for example spacers. Thiscan be a time consuming process.

It is an object of an exemplary embodiment to obviate or mitigate theproblems outlined immediately above.

According to a fourth aspect of an exemplary embodiment, there isprovided a printing apparatus comprising a housing, a print head mountedon a print head support assembly which is displaceable relative to thehousing in a direction parallel to a print ribbon path, a first drivemechanism for displacing the print head support relative to the housing,a roller which in use supports a substrate to be printed on the side ofthe ribbon path remote from the print head, a second drive mechanism fordisplacing the print head relative to the print head support assembly toa printing position in which a portion of the print head bears againstthe roller or any substrate or ribbon interposed between the print headand roller, and a controller for adjusting the first drive mechanism toadjust the angular position of the print head relative to the rotationaxis of the roller.

Preferably, the portion of the print head that bears against the rolleror any substrate or ribbon interposed between the print head and roller,is the portion of the print head that contains selectively energizeableprinting elements. The elements may be linearly arranged along theportion of the print head, for example the linear array of elements maybe arranged along an edge, or parallel in close proximity to an edge ofthe print head.

In operation, an installer could initially position a print head so thatit would assume a nominal position which would be expected to producegood quality print. A test print run would then be used to assess printquality, the print head support would then be displaced relative to thehousing, and a fresh print run would be conducted, the process beingrepeated until the resultant print quality was optimized. There is thusno requirement for the installer to make small mechanical adjustments tothe position of the print head on its support.

The printing apparatus in accordance with the fourth aspect of theexemplary embodiment may be used in conjunction with any of the featuresof the first, second and third aspects of the exemplary embodiment asdefined above.

In many tape drive mechanisms and particularly in ribbon printingmachines, loading a fresh print ribbon can be a difficult process as theprint ribbon has to be correctly positioned along a non-linear path.Often replacement print ribbons are mounted in a cassette which isdesigned to be readily mounted in a predetermined orientation on anassociated printing apparatus. In such arrangement it is generallynecessary to position a length of ribbon extending between support onthe cassette between a print head and a peel off roller. This isdifficult to achieve unless the print head and peel off roller can bemoved apart to provide a wide enough track into which the ribbon can beinserted.

It is known to provide an arrangement in which either the print head orthe peel off roller can be displaced by a lever mechanism which isactuated when a cassette is mounted on a printing apparatus. For exampleif the cassette is held in position by a mechanical latch, release ofthe latch will move the print head and peel off roller apart whereasengagement of the latch moves them together to a ready-to-printposition.

Such arrangements are satisfactory in terms of performance butdisadvantageous as valuable space is occupied by the lever mechanisms,thereby reducing the space available for taking large diameter spools oftape.

It is an object of an exemplary embodiment to obviate or mitigate theproblems outlined above.

According to a fifth aspect of an exemplary embodiment, there isprovided a printing apparatus comprising a housing, a print head mountedon a print head support assembly which is displaceable relative to thehousing in a direction parallel to a print ribbon path, a first drivemechanism for displacing the print head support relative to the housing,a peel off roller mounted on the print head support assembly anddisplaceable with the print head in the said parallel direction, and asecond drive mechanism for displacing the print head relative to theprint head support assembly and peel off roller between a ready to printposition adjacent the print ribbon path and a printing position in whichthe print head would contact a print ribbon on the path, wherein a cammechanism is provided which is engaged as a result of displacement ofthe print head support assembly to a predetermined position and whenengaged causes retraction of the print head away from the ready to printposition to a position spaced from the peel-off roller and the printribbon path.

In an arrangement in accordance with the fifth aspect of an exemplaryembodiment, when a cassette carrying a print ribbon is to be replaced,an electronic signal can be generated to cause transport of the printsupport assembly to a predetermined position (a “docked” position). Thisautomatically retracts the print head away from the peel-off roller,enabling the easy insertion of a tape between the print head andpeel-off roller.

The printing apparatus according to the fifth aspect of the exemplaryembodiment may be used in conjunction with any of the features of thefirst, second, third and fourth aspects of the exemplary embodiment asdefined above.

Another problem encountered with printing machines is that of achievingsufficient tape transport speeds in the interval between printing phasesof successive printing cycles. In some instances the time taken totransport a printing ribbon by a distance equal to the length of ribbontraversed by the printing head during one printing cycle is a limitingfactor in overall machine speed. It would be advantageous to be able toreduce the distance that a ribbon is advanced between any two successiveprinting cycles.

According to a sixth aspect of an exemplary embodiment, there isprovided a printing apparatus comprising a print head, a printing ribbondrive mechanism for advancing a printing ribbon between the print headand a path along which in use a substrate to be printed is advanced,means for applying the print head to a ribbon supported in the drivemechanism, the print head comprising an array of printing elements eachof which may be selectively energized to release ink from a portion ofribbon contacted by that element, and a controller for controllingenergization of the printing elements and the advance of the ribbon soas to perform a series of printing cycles each of which includes aprinting phase during which relative movement between the print head andribbon results in the print head reversing a predetermined length ofribbon and a non-printing phase during which the ribbon is advanced apredetermined distance relative to the print head, herein the controlleris arranged selectively to energize different groups of printingelements during successive printing cycles, the groups of elements beingdistributed on the print head such that different groups contactdifferent portions of the ribbon, and the controller is arranged toadvance the ribbon such that the said predetermined distance of ribbonadvance is less than the said predetermined length of ribbon, the groupsof printing elements being energized such that the ribbon is advanced byat least said predetermined length of ribbon in the interval between anytwo printing phases in which the same group of printing elements areenergized.

If the printing elements are arranged in two groups, for examplealternate pixels distributed across a linear printing head, an image maybe printed in one printing cycle using one group, the ribbon may beadvanced by half the length of ribbon traversed by the printer duringthe first cycle, a second image may be printed using the other groupduring a second cycle, the ribbon may again be advanced by half thetraverse distance of the printing head, and then the first group may beenergized during a third printing cycle and so on. Thus the maximum tapetravel between successive printing cycles can be half that which must beaccommodated in conventional printing systems.

The printing apparatus according to the sixth aspect of the exemplaryembodiment may be used in conjunction with any of the features of thefirst, second, third, fourth and fifth aspects of the exemplaryembodiment as defined above.

It will of course be appreciated that if the printing elements weredivided into three groups, tape advance between successive cycles couldbe limited to one third of the length of ribbon traversed by the printhead in one cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a printer ribbon drive system inaccordance with the exemplary embodiment;

FIG. 1 a is an illustration of a modification to the drive system ofFIG. 1;

FIG. 2 is a perspective view of a printer drive assembly of a ribbondrive system as described with reference to FIG. 1;

FIG. 3 is a schematic perspective view of a printer ribbon cassettewhich may be mounted on the assembly of FIG. 2;

FIGS. 4 to 9 are further illustrations of the drive assembly of FIG. 2;

FIG. 10 is a perspective view of a print head support carriageincorporated in the drive assembly of FIG. 2;

FIG. 11 is a perspective view of an alternative print head supportcarriage to that shown in FIG. 10 which may be used to reverse thepositions of components of the drive assembly of FIG. 2;

FIG. 12 is a view of the drive assembly after conversion using thealternative print head support of FIG. 11;

FIGS. 13 to 16 illustrate the use of interleaved printing using thedrive assembly of FIG. 2;

FIG. 17 schematically illustrates the operation of an optical printerribbon spool diameter measuring system;

FIG. 18 is a schematic illustration of a circuit for monitoring thepower consumed by stepper motors incorporated in the drive assembly ofFIG. 2;

FIG. 19 is a schematic illustration of a circuit for monitoring thecharging ratio between the diameters of ribbon spools mounted on thedrive assembly of FIG. 2;

FIG. 20 illustrates an alternative approach to monitoring ribbon spooldiameters;

FIG. 21 illustrates the adjustment of print head angle in accordancewith the exemplary embodiment; and

FIG. 22 illustrates the use of an apparatus in accordance with theexemplary embodiment to produce images while relying upon only limitedprinter ribbon advance.

DETAILED DESCRIPTION

Referring to FIG. 1, the schematically illustrated printer in accordancewith an exemplary embodiment has a housing represented by broken line 1supporting a first shaft 2 and a second shaft 3. A displaceable printhead 4 (PR Head) is also mounted on the housing, the print head beingdisplaceable along a linear track as indicated by arrows 5. A printerribbon 6 extends from a spool 7 received on a mandrel 8 which is drivenby the shaft 2 around rollers 9 and 10 to a second spool 11 supported ona mandrel 12 which is driven by the shaft 3. The path followed by theribbon 6 between the rollers 9 and 10 passes in front of the print head4. A substrate 13 upon which print is to be deposited follows a parallelpath to the ribbon 6 between rollers 9 and 10, the ribbon 6 beinginterposed between the print head 4 and the substrate 13.

The shaft 2 is driven by a stepper motor 14 (SM) and the shaft 3 isdriven by a stepper motor 15 (SM). A further stepper motor 16 (SM)controls the position on its linear track of the print head 4. Acontroller 17 (Contr) controls each of the three stepper motors 14, 15and 16 as described in greater detail below, the stepper motors beingcapable of driving the print ribbon 6 in both directions as indicated byarrow 18.

In the configuration illustrated in FIG. 1, the spools 7 and 11 arewound in the same sense as one another and thus rotate in the samerotational direction to transport the tape. FIG. 1 a illustrates amodification of the drive system of FIG. 1 in which the spools are woundin the opposite sense to one another and must rotate in oppositedirections to transport the tape. Thus the first spool 7 rotatesclockwise while the second spool 11 rotates anticlockwise to transportthe printer ribbon 6 from the first spool 7 to the second spool 11.

As described in greater detail below, the printer schematicallyillustrated in FIG. 1 can be used for both continuous and intermittentprinting applications. In continuous applications, the substrate 13 willbe moving continuously. During a printing cycle, the print head will bestationary but the ribbon will move so as to present fresh ribbon to theprint head as the cycle progresses. In contrast, in intermittentapplications, the substrate is stationary during each printing cycle,the necessary relative movement between the substrate and the print headbeing achieved by moving the print head during the printing cycle. Theribbon generally will be stationary during the printing cycle. In bothapplications, it is necessary to be able to rapidly advance and returnthe ribbon 6 between printing cycles so as to present fresh ribbon tothe print head and to minimize ribbon wastage. Given the speed at whichprinting machines operate, and that fresh ribbon should be presentbetween the print head and substrate during any printing cycle, it isnecessary to be able to accelerate the ribbon 6 in both directions at ahigh rate and to accurately position the ribbon relative to the printhead. In the arrangement shown in FIG. 1 it is assumed that thesubstrate 13 will move only to the right as indicated by arrow 19 but asdescribed below the apparatus can be readily adapted to print on asubstrate traveling to the left in FIG. 1.

Referring to FIGS. 2, 3 and 4, electromechanical components making upthe printer shown in outline with reference to the schematicillustration of FIG. 1 will now be described. The printer housing 1comprises a casing 20 in which various electronic components to bedescribed below are housed behind a cover plate 21. The shafts 2 and 3project through apertures in the cover plate 21, guide pins 9 a and 10 aproject from the cover plate 21, and the print head 4 is mounted abovethe cover plate 21. The print head 4 is part of an assembly which isdisplaceable along a linear track 22 which is fixed in position relativeto the cover plate 21. The stepper motor 16 which controls the positionof the print head assembly is located behind the cover plate 21 butdrives a pulley wheel 23 that in turn drives a belt 24 extending arounda further pulley wheel 25, the belt being secured to the print headassembly. Thus rotation of the pulley wheel 23 in the clockwisedirection in FIG. 4 drives the print head assembly to the left in FIG. 4whereas rotation of the pulley wheel 23 in the anti-clockwise directionin FIG. 4 drives the print head assembly to the right in FIG. 4. Thepulley wheels 23 and 25 and the linear track 22 are mounted on a rigidbracket 26 which extends upwardly from the cover plate 21. FIG. 2 showsdrive discs mounted on the shafts 2 and 3, the drive discs definingdiametrically spaced sockets intended to engage ribbon spools 7 and 11,whereas in FIG. 4 the drive discs have been removed to show the uppersurfaces of the stepper motors 14 and 15.

Referring to FIG. 3, this illustrates a printer ribbon supported on acassette which may be mounted on the printer of FIG. 2. Hollow rollersupports 9 b and 10 b are intended to receive the pins 9 a and 10 arespectively shown in FIG. 2, such that the combination of pin 9 a andhollow roller 9 b together constitute the roller 9 of FIG. 1 and suchthat the pin 10 a and hollow roller 10 b together constitute the roller10 of FIG. 1. The spools 7 and 11 are supported on the mandrels 8 and 12which are a push fit on rotatable shafts mounted on a common cover platewith the hollow rollers 9 b and 10 b. The rotatable shafts define pinswhich engage with the sockets defined on the drive discs driven by theshafts 2, 3. Thus, with the cassette in place, the ribbon can betransferred between the two spools 7 and 11.

The housing cover plate 21 (FIG. 2) also supports an upstanding rearbracket 27 on which a pair of emitters 28, 29 are supported. These twoemitters operate in cooperation with a receiver which is displaceablewith the print head assembly as described in greater detail below.

The print head assembly 4 is shown in a “docked” position in FIGS. 2 and4 and in a position in FIG. 5 in which it is ready-to-print on a rollerplaten 30 (assuming operation in a continuous mode with a continuouslymoving substrate), and in a ready to print position in FIG. 6 in whichthe print head is ready to print on a substrate which is stationary andpositioned in front of a stationary flat platen 31. In the positionshown in FIGS. 2 and 4, an edge 32 of the print head 4 is retractedbehind the ribbon path between rollers 9 and 10 whereas a peel-offroller 33 is positioned on the opposite side of the ribbon path from theprint head 4. This makes it an easy matter to install a fresh cassetteof ribbon. In contrast, in the ready-to-print positions shown in FIGS. 5and 6, the print head 4 has been advanced so that the edge 32 projectsjust beyond the outer extremity of the roller 33. Thus in theready-to-print position the print ribbon passes around the edge 32 andis deflected away from the underlying substrate by the peel-off roller33.

The edge 32 of the print head 4 (which is of conventional form) supportsan array of heating elements each of which is selectively energizeable.When the ribbon 6 is sandwiched between the head 4 and a substrate 13,ink adjacent any energized heating element is melted and transferred tothe substrate. Thus by appropriate control of the heating elements,small portions of ink carried by the ribbon 6 can be transferred to thesubstrate 13. Each of these portions of ink can be considered asdefining one pixel of the image to be printed.

Referring now to all of FIGS. 2 to 9, the detailed structure of theprint head assembly and the slider upon which it is mounted will bedescribed. FIG. 9 shows the print head assembly pulled forward to anadjustment position revealing associated components of the assembly.FIG. 9 is the best view of a slot 34 formed in the upstanding bracket 26on which the linear track 22 is mounted. A slider 35 supporting a printhead carriage 36 is mounted on the linear track 22 (FIG. 2). The slider35 and track 22 are high-accuracy products to provide smooth, lowfriction, parallel movement of the print head carriage 36 relative tothe bracket 26. An optical detector 37 is mounted on the print headcarriage 36 so as to be in register with the slot 34 formed in thebracket 26. The detector 37 is used as described below to detect lightemitted from the emitters 28 and 29, the slot 34 ensuring that the onlyobstruction between the detector 37 and the emitters 28 and 29 will beany spools of ribbon mounted on the printer in a cassette such as thatillustrated in FIG. 3. The cassette is secured against displacementrelative to the components illustrated in FIG. 3 by a permanent magnet(not shown) incorporated in the cassette body and cooperating with acircular steel keeper 38 mounted on top of the bracket 26 (FIG. 4).Alternative arrangements for securing the cassette in position are ofcourse possible, for example mechanical latch assemblies.

The print head carriage 36 supports the print head assembly whichcomprises the print head 4 which is bolted to a pivotal plate 39 that ismounted to pivot about pivot pin 40 that in turn is mounted on a plate41 bolted to the print head carriage 36. A spring 42 biases the plate 39towards the plate 41 so that in the absence of any obstruction the printhead 4 will assume the position relative to the print head carriage 36as shown in FIG. 4. The peel off roller 33 is fixed in position on anarm 43 which is bolted to the print head carriage 36.

A pneumatic drive unit 44 is a sliding fit within a slot provided in theprint head carriage 36 and drives a piston 45 which is shown in theextended position in FIG. 8 and the retracted position in FIG. 7. Thepneumatic drive 44 is connected to a flexible pneumatic supply line (notshown) connected to an air inlet 46 (FIG. 2). The inlet 46 is connectedto a tube 47 which extends through an opening in the print head carriage36 so as to communicate with the pneumatic drive unit 44. The pneumaticdrive unit piston 45 bears against a U-shaped member 48 which is coupledby pivot pin 49 to a U-shaped bracket 50. The bracket 50 supports a pin51 (FIG. 9) which is intended to engage in a slot 52 in a cam plate 53.The bracket 50 defines a curved corner 54 which is intended to engageagainst curved surface 55 defined in plate 39 as shown in FIGS. 7 and 8.If however the pin 51 is received in and pushed to the blind end of theslot 52, the bracket 50 is pushed away from the print head 4, enablingthe plate 39 to swing towards the plate 41 so that the print head 4assumes the docked position shown in FIGS. 2 and 4.

The bracket 50 is spring biased by a spring (not shown) coupled to alever 50 a (see FIG. 7) so as normally to assume the position shown inFIG. 7. If pressurized air is then supplied to the pneumatic drive 44,the assembly assumes the position shown in FIG. 8 in which it will beseen that the printing edge 32 of the print head 4 has been pushed wellbeyond the peel-off roller 33. If with the pneumatic drive unit 44de-energized and therefore the U-shaped member 48 in the position shownin FIG. 7 the carriage is moved so that the pin 51 enters the slot 52,further movement of the carriage in the same direction will cause thepin 51 to move into the blind end of the slot, thereby causing thebracket 50 to turn about the pivot pin 49 so as no longer to obstructmovement of the print head 4 to the docked position. If movement of thecarriage is then reversed, the pin 51 causes the bracket 50 to swing outagain, pushing the print head 4 to the position shown in FIG. 7. Theposition shown in FIG. 7 corresponds to “ready to print” and theposition shown in FIG. 8 corresponds to “printing”.

FIG. 10 is a perspective view of the print head carriage 36 showing thesocket which in the assembled apparatus receives the pneumatic driveunit 44. An opening 56 is provided to receive the air inlet tube 47 (seeFIG. 7). A tongue 57 projects from the lower edge of the print headcarriage 36 and is used in a manner not shown to attach the print headcarriage to the belt 24.

In the exemplary embodiment as illustrated in FIGS. 1 to 10, it isintended that a substrate to be printed travels past the print head inthe left to right direction with respect to FIG. 5 or the print headwhen printing travels in the right to left direction with respect to theplaten 31 in FIG. 6. The peel-off roller 33 is in all instancespositioned on the downstream side of the printing edge 32. There aremany circumstances however where such an arrangement is not convenientand it would be desirable to reverse the arrangement so that therelative positions of the edge 32 and the peel off roller 33 arereversed and the disposition of the print head 4 is also reversed. Thiscan readily be achieved with the illustrated apparatus by replacing theprint head carriage 36 shown in FIG. 10 with the print head carriage 58shown in FIG. 11. FIG. 12 illustrates the resultant assembly. It will benoted that the print head carriage 58 of FIG. 11 also defines a socket59 for receiving the pneumatic drive unit 44 and an opening 60 forreceiving the air inlet tube 47. It will also be noted that the printhead carriage 58 of FIG. 11 is a mirror image about a vertical plane ofthe print head carriage 36 of FIG. 10.

Referring to FIG. 12, it will be seen that in addition to reversing theposition of the print head 4 and the peel off roller 33, the cam plate53 has also been rotated through 180° and fitted on the opposite side ofthe magnet 38 (circular steel keeper) to its position in the embodimentof FIGS. 1 to 10. The arm 43 on which the peel off roller 33 is mountedhas also been moved so as to continue to be located adjacent the coverplate 21.

The described printer arrangement provides a number of very significantadvantages. Firstly, it is possible to use the same apparatus for bothcontinuous and intermittent printing. Conversion of a production linefrom one form of printing to another does not therefore mean that newprinters must be purchased. Secondly, by making relatively minormodifications involving only one additional component (the alternativeprint head carriages of FIGS. 10 and 11) the same apparatus can be usedfor both left hand and right hand applications, using these terms in thesense of FIG. 2 or FIG. 4 (left hand) and FIG. 12 (right hand). Thirdly,ribbon replacement is simple matter given that when in the dockedposition the print head 4 is automatically pulled back away from thepeel-off roller 33 so as to provide a wide track into which areplacement printer ribbon carried on a cassette can be inserted.

Referring to FIGS. 13, 14, 15 and 16, different methods of makingefficient use of the printer ribbon using the apparatus described inFIGS. 1 to 12 will be described. All of these methods rely upon the highaccuracy within which ribbon can be delivered to the print head so as tominimize ribbon wastage.

Referring to FIG. 13, this is a view of a ribbon the length of which isindicated by arrow 61 and with which six individual printing operationshave been performed using overlapping regions of the ribbon. These sixregions are indicated as regions 62 to 67, the second half of region 62overlapping with the first half of region 63, the second half of region63 overlapping with the first half of region 64 and so on. Assumingprinting on a substrate, the region 62 is printed, the ribbon is thenadvanced by half the length of the regions, the region 63 is printed,the ribbon is then again advanced by half the length of the regions, theregion 64 is then printed and so on.

Such overlapping printed regions could be used in both continuous andintermittent printing processes. In the described arrangement, adjacentregions overlap by half the width of each region, but differentproportions of overlap could be envisaged. Given that adjacent printingregions overlap, it is important that a region of the ribbon which isoverlapped by two adjacent printing regions is used in a manner whichensures that printing progresses only on the basis of using portions ofthe ribbon which are used in only one of the two overlapping regions.This can be achieved for example by selecting only alternate portions ofthe ribbon within any one printing region. For example, as illustratedin FIG. 14, if adjacent heating (pixel) elements on the printing headare represented by ribbon areas 68 and 69, ribbon areas 68 would be usedin printing one region (for example region 62) and ribbon areas 69 wouldbe used in printing the adjacent region (region 63). In this manner,providing the spacing between adjacent pixels on the print head is smallenough to enable an image of reasonable quality to be printed using onlyalternate pixels, twice the number of images can be generated from aribbon than would be the case if all the pixel elements were used forprinting purposes in a single image and there was no overlap betweenprinting regions. In addition however the distance that the ribbon mustbe advanced between printing phases in successive printing cycles isreduced by half. This is advantageous as in some applications thisenables faster machine operation.

To illustrate this advantage, FIG. 15 shows conventional printing onto asubstrate with no overlap between successive cycles whereas FIG. 16illustrates the same operation relying upon such overlap.

Referring to FIG. 15, a substrate 70 is shown on which successive images71 and 72 have been printed. Shown beneath the substrate is a printribbon 73 on which areas 74 and 75 have been used to produce the images71 and 72. The ribbon transport length is indicated by the arrow 76 andis equal to twice the length of a single image.

Referring to FIG. 16, this shows how overlapping printing can bothreduce ribbon usage and reduce the distance of ribbon transport betweensuccessive printing phases. It will be seen that each of the areas 74and 75 in FIG. 16 is only half the length of the corresponding areas inFIG. 15 and the ribbon transport distance is therefore halved. In someapplications, where rapid ribbon transport is required, halving thedistance that ribbon must be transported between successive printingphases can significantly improve the ability of the device to operate athigh speed. It will also be appreciated that more than two groups ofprinting elements may be used so that in the case of for example threegroups the length of required ribbon transport would be only one thirdof the image length. Thus there would be a trade off between printerribbon transport length and image quality but this aspect of theexemplary embodiment does give the operator of such equipment increasedflexibility which in some applications will be of real economicsignificance.

The advantages described with references to FIGS. 13 to 16 can only beachieved if the print ribbon can be positioned relative to the substrateand the print head with great accuracy. The conventional approach toachieving accurate control of tape acceleration, deceleration, speed andposition has relied upon a capstan roller positioned between feed andsupply spools, but the exemplary embodiment relies upon a completelydifferent approach, that is the accurate control of the drive applied tothe stepper motors 14 and 15 (FIG. 1) which drive the ribbon spools. Thestepper motors operate in push-pull bi-directional mode, that is if thetape is traveling in one direction between the spools both steppermotors are driven in that direction, and conversely when the ribbon isbeing driven in the opposite direction both stepper motors are driven inthat opposite direction. Coordination of the drive to the two steppermotors requires knowledge of the diameters of the spools and this isachieved using the light emitting devices 28 and 29 and the lightdetecting device 37 as shown in for example FIG. 2.

FIG. 17 illustrates how the light emitting devices 28 and 29 and thedetector 37 are used to determine the spool diameters. The detector 37is mounted on the print head carriage 36 and is displaceable between theposition indicated by line 76 a and the position indicated by line 77.As the detector 37 is moved to the right in FIG. 17 from the positionindicated by line 76, initially emitter 28 is energized. Initially thedetector 37 is in the shadow cast by spool 7, but as soon as thedetector 37 crosses the plane indicated by line 78 a an output will begenerated. That output will disappear as the detector 37 crosses theplane indicated by line 78 b. The detector 37 is then advanced to theposition indicated by line 77 and then returned after the emitter 28 hasbeen de-energized and the emitter 29 has been energized. Initially thedetector 37 will be in the shadow of spool 11 but will generate anoutput as soon as it reaches the plane indicated by the line 79 a. Thatoutput will disappear as the detector 37 crosses the plane indicated bythe line 79 b. The positions relative to the detector displacement atwhich the detector 37 intersects the planes 78 a, 78 b, 79 a and 79 bcan thus be determined. The dimension A, that is the distance betweenthe rotation axes of the two spools, is known. The perpendiculardistance B between the track followed by the detector 37 and the planein which the emitters 28 and 29 are located is known, as is theperpendicular distance C from the axes of the shafts 2 and 3 to thetrack followed by the detector 37. From these dimensions the diametersD1 and D2 of spools 7 and 11 can be derived using simple trigonometry.

Two emitters 28, 29 are used to ensure that for any one spool thedetector 37 can “see” the shadow cast by at least one of the emittersregardless of spool diameter size. It will be appreciated however thatother dispositions of one or more emitters and one or more detectorscould be envisaged.

It will be appreciated that the calculation of the spool diameters wouldbe somewhat simpler if the planes 78 a, 78 b, 79 a and 79 b wereperpendicular to the direction of displacement of the detector 37. Thiscan be achieved by for example replacing the emitters 28 and 29 with amirror extending parallel to the direction of displacement of the printhead carrier 36 and arranging both a transmitter and a detector on theprint head carriage 36, the detector detecting light only when both itand the emitter are on a plane perpendicular to the mirror. Althoughsuch an arrangement is simple in terms of the required trigonometry ithas disadvantages in that a failure of either the transmitter ordetector could be interpreted as the detector being in the shadow of oneof the spools.

Given knowledge of the spool diameters, the spools can be driven inpush-pull mode so as to achieve high rates of acceleration anddeceleration by appropriate control of the speeds of rotation of the twostepper motors. Tension in the ribbon between the two spools musthowever be closely controlled to avoid the tension becoming too high(resulting in over tightening of the ribbon on the spools or even ribbonbreakage) or the tension becoming too low (resulting in loss ofpositional control as a result of the ribbon becoming slack). To avoidthis occurring, changes in spool diameters over time are monitored byreference to the stepper motors and tension in the ribbon is directlymonitored by reference to the current drawn by the stepper motors.

In one embodiment, when a fresh cassette is fitted onto an apparatussuch as that described with reference to FIGS. 1 to 10, one of thecassette shafts will support an almost empty spool (the take-up spool)and the other will support an almost full spool (the supply spool). Thestepper motor associated with the take-up spool will be referred tobelow as the take-up motor and the other stepper motor will be referredto as the supply motor.

Initially the take-up motor is energized to remove any slack from thelength of ribbon extending between the two spools. A print head scan isthen conducted with the optical system described with reference to FIG.17 to obtain an initial estimate of the diameters of the spools. Thesupply motor is then energized in order to tension the ribbon extendingaround the supply spool. The take-up motor is then driven so as to drawribbon from the supply spool, the supply spool being de-energized. Thenumber of steps taken by the motor driving the take-up spool ismonitored. The other motor is not stopped, but generates a back-emfresulting in the generation of pulses that are counted. After a fewturns of the spools the number of steps taken by the take-up motor andthe number of pulses generated by the supply spool motor are counted andthe counted numbers are used to establish the ratio between the twodiameters. The ribbon is then brought to a controlled halt. Both motorsare decelerated in a controlled manner to avoid overrun. Thus the supplyspool motor is driven by pulses to cause deceleration. The applicationof deceleration pulses to the supply spool motor in synchronism withmotor rotation is achieved by monitoring the back-emf generated in onewinding of that motor, and then energizing that winding at anappropriate time to apply a decelerating torque. A number of rotationsof the take-up spool are required to minimize the chance of any tails ofribbon extending from the spools obstructing the optical paths of thescanning arrangement as illustrated in FIG. 17. A further optical scanis then performed in both directions to determine the radius of thetake-up spool while that spool is stationary. An optical scan is thenrepeated as the spool is rotated in 30° increments around the steppermotor shaft by stepping the motor by the appropriate number of steps,that number being a constant. This builds up a map of the dimensions ofthe spool (which may not be perfectly circular) and this map is used tocalculate the average radius for each spool for the arc that each willrotate in each ribbon feed and further use these radii to calculatevariations in diameter around the spool axes. This makes it possible toaccurately determine the circumference of each spool and the effect of apredetermined number of steps in advance of the motor driving thatspool. For example the different calculated radii can be used tocalculate the step rate and the number of steps required by each motorto drive the spools in an appropriate manner so as to feed the ribbon apredetermined distance. These radii and step rates may then be used intension monitoring calculations such as those described below.

The same optical scan procedure is then performed in both directions tomeasure the radius of the supply spool. This information is thencombined with the previously calculated ratio of spool diameters to givean accurate set of data related to the spool diameters and shapes.Ribbon fed from the supply spool to the take-up spool is then rewoundback onto the supply spool so as to avoid ribbon wastage.

Stepper motors generally comprise two quadrature-wound coils and currentis supplied in a sequence of pulses to one or both of the coils and inboth senses (positive and negative) so as to achieve step advance of themotor shafts. In order to achieve a reasonable performance despite theinherent electrical time constant of these coils it is well known toover-drive stepper motors by applying a voltage that is much larger thanthe nominal rating of the motor and to pulse width modulate this voltagewhen the desired motor current is reached. For example, with a 3.6 voltmotor capable of taking say 2 amps, a voltage of 36 volts may beapplied. This results in a very rapid rise in current through the motor,typically in a few tens of micro seconds. Given such overdriving of thesupply voltage, relatively short periods of supply voltage applicationare separated by relatively long periods during which no supply voltageis applied. As a result current from the supply to the motors is veryfar from smooth. In addition, even when a motor is operating with zeroload relating to the function that it performs (equating to zero tensionin the printer ribbon), the supply current to the motor will be afunction of various factors such as the speed of rotation of the motor,the particular characteristics of that motor (efficiency etc.), and theparticular characteristics of the motor drive circuitry (gain and offsetvariances). It is necessary therefore to calibrate the motors to takeaccount of current variation related to these factors rather than motorload.

The motors are calibrated by driving each of them in zero-loadconditions at each of a series of different speeds, for example atspeeds corresponding to 125 steps per second, 250 steps per second, 375steps per second and so on in increments of 125 steps per second up to5000 steps per second. This will generally cover the range of ribbonspeeds required for ribbon advancement, that range generally being from100 mm per second to 600 mm per second ribbon transport speed. Thisprocess is repeated a number of times, for example twenty times, and theaverage result is used to calculate a motor calibration factor x foreach step rate, and for each motor. The following relationship is used:x=N/V

Where:

x is the calibration factor for the motor at the given step rate.

V is the average measured motor operation value at the given step rate.

N is a constant normalization or scaling factor.

From the above for each motor a series of values x is calculated foreach of the predetermined step rates. When the apparatus is in use, fora given step rate one of the values x is selected for use in thecalculation of ribbon tension, or a value for x is calculated for thegiven step rate by interpolation from the two values of x for thepredetermined step rates closest to the given rate.

FIG. 18 illustrates the calculation of the values V both during motorcalibration and in subsequent ribbon tension control. Referring to FIG.18, a regulated power supply 80 energizes a first motor drive circuit 81(MDC) and a second motor drive circuit 82 (MDC). Current from the supply80 to the motor drive circuit 81 is delivered through a low resistanceresistor 83, the potential developed across the resistor 83 beingapplied to a level translator 84 (LT). Similarly, current to the motordrive 82 is delivered through a low resistance value resistor 85 and thevoltage developed across that resistor is applied to a level translator86 (LT). The outputs of the level translators 84 and 86 are applied toanalog to digital converters 87 and 88 (ADCs) the outputs of which areapplied to a microcontroller 89. The microcontroller delivers a pulsedoutput 90 to the first motor drive 81 and a pulsed output 91 to thesecond motor drive 82. The motor drives energize stepper motorsschematically represented by cylinders 92 and 93 which drive respectivespools 94 and 95.

During motor calibration, no spools are mounted on the outputs of thestepper motors 92 and 93. For a given step rate for each motor theoutputs of the ADCs 87 and 88 are recorded such that x and V for eachmotor at each of the preselected step rates is known. Those values arethen used as described below to enable direct monitoring of ribbontension in the ribbon between the spools 94 and 95, these spools havingbeen mounted on the output shafts of the stepper motors 92 and 93.

The formulas used for tension calculation are as follows, assuming thatmotor 92 is pulling and motor 93 is pushing:V ₁ x ₁=(N+r ₁ tx ₁)f(T)   (1)V ₂ x ₂=(N−r ₂ tx ₂)f(T)   (2)

Where:

V₁ is the output of ADC 88 given a selected constant step-rate ribbonfeed

V₂ is the output of ADC 87 during ribbon feed

r₁ is the radius of the spool 94

r₂ is the radius of the spool 95

x₁ is the calibration factor for motor 92 for the selected constant steprate

x₂ is the calibration factor for motor 93 for the step rate of motor 93

N is the scaling factor used during motor calibration

t is the ribbon tension

f(T) is a temperature-related function

Temperature variations which will affect the measured values V₁ and V₂will generally affect both motors to the same extent. Therefore bydividing equation (1) by equation (2) the functions f(T) will cancelout. The equation can therefore be resolved to derive a measure oftension t as follows:t=N((V ₁ /x ₂)−V ₂ /x ₁))/(V ₂ r ₁ +V ₁ r ₂)   (3)

Thus for any given step rate for the motors, the appropriate calibrationfactors x₁, x₂ can be looked up and used to derive a measure of theribbon tension t. If the derived value of t is too high (above apredetermined limit), then a small step adjustment can be made to eitheror both of the motors to add a short section of ribbon to the length ofribbon between the spools. If the derived value of t is too low (below adifferent predetermined limit), then a short section of ribbon can beremoved from the length of ribbon between the spools. The controlalgorithms used to determine the correction amounts of ribbon added toor removed from the length of ribbon between the spools may be ofconventional form, for example the algorithms known as proportionalintegral derivative control algorithms (PID control). The algorithmsmake it possible to compare the measured tension t with predeterminedupper and lower limits (the so-called deadband) and, if the measuredtension is outside these limits, the difference between the measuredtension t and a “nominal demand” tension which is set at a level betweenthe upper and lower limits may be calculated, the result of thatcalculation being regarded as an error “signal”. This error “signal” isthen mathematically processed through the PID algorithms, which includea proportional gain constant, as well as integral and derivativefactors. The mathematical processing results in a “correction” amount ofribbon that needs to be added to or removed from the ribbon path betweenthe spools during the next ribbon feed. This addition or removal ofribbon maintains ribbon tension within acceptable limits.

In greater detail, the correction value may be calculated by calculatingthe error (the difference between the nominal tension and the measuredtension) and dividing the error by a gain factor which depends upon theribbon width. The greater the gain factor the tighter the system will beas the nominal tension will be increased. The gain factor is alsodependent upon the ribbon width as the gain constants are changed totake account of different ribbon widths. This is because a tension whichmight cause considerable stretch in a narrow ribbon would cause minimalstretch in a wide ribbon and therefore the effects of adding or removingribbon from the length of ribbon between the spools is radicallyaffected by ribbon stiffness. Successive cycles may adjust the gainfactor from a value nominally of 100 (tight) to a value of nominally 80(slack). For every consecutive tight or slack reading after a firstreading, an extra 0.1 mm correction can be added. An error accumulatoris also maintained, and if the accumulated corrections (which arenegative for tight and positive for slack) exceed plus or minus 2 mmthen an additional 0.1 mm is added to the correction. These are the twointegral components which enable the system to operate in a stablemanner and maintain ribbon tension at or close to the nominal tension.

The motor feed system splits the correction evenly between both motorsin order to avoid large gaps between prints or over-printing on theribbon. The system does this by calculating the number of steps thathalf the correction amounts to for the stepper motor with the largestreal diameter. These steps are then re-calculated as a distance (relyingupon the known spool diameters) and subtracted from the originalcorrection amount. The resultant value is then used to calculate thecorrection for the motor driving the smaller diameter spool. Because themotor driving the smaller diameter spool has the smallest step size(when each step is converted to ribbon length) it can most accuratelyfeed the remaining distance. Thus the mechanism adjusts the tension byan amount that is as near as possible to that demanded by the originalcorrection.

It will be appreciated that if a particularly low tension reading iscalculated by the above method, this can be taken by the control systemas indicating a fault condition, for example ribbon breakage, or theribbon becoming so slack that the system is most unlikely to be able toeffect adequate control. In such circumstances, the control system canoutput a “broken ribbon” predetermined low limits, such that when themeasured tension t falls below this limit, the control system can haltthe printing process and assert appropriate fault outputs and warningmessages. Thus the system can offer valuable “broken ribbon” detectionwithout the need for additional sensing arrangements.

FIG. 19 illustrates a circuit for calculating the ratio of the diametersof the spools 94 and 95 in the circuit of FIG. 18. The positive supplyrail 96 of the power supply 80 (FIG. 18) is arranged to supply currentto four windings 97, 98, 99 and 100. Current is drawn through thewindings 97 to 100 by transistors 101 which are controlled by motorcontrol and sequencing logic circuits 102. The step rate is controlledby an input on line 103 and drive is enabled or disabled by an input online 104 (high value on line 104 enables, low value disables). Asbefore, if motor 92 is pulling, the drive circuit 108 for that motor isenabled and therefore the rotation angle for the spool being driven (94)is known. The drive circuit for the motor being pulled (93) is disabled(line 104 low). Thus motor 93 acts as a generator and a back-emf isgenerated across each of the motor windings 97 to 100. The componentsenclosed in box 108 of FIG. 19 corresponds to one of the motor drivecircuits 81, 82 of FIG. 18. The voltage developed across the winding 100is applied to a level translator circuit 105 (LT) the output of which isapplied to a zero crossing detector 106 fed with a voltage reference onits positive input. The output of the zero crossing detector 106 is aseries of pulses on line 107. Those pulses are delivered to the microprocessor 89 of FIG. 18. By counting these pulses from motor 93 over aknown rotation angle of the drive motor 92 the spool diameter ratio canbe calculated.

The method of monitoring ribbon tension as described with reference toFIG. 18 relies upon sampling current supplied to the motor drives 81 and82 by sampling voltages developed across series resistors 83 and 85.Preferably current is detected only during periods in which the ribbonhas been advanced at a constant speed. In intermittent printing systems,current is monitored during the return stroke of the print head aftereach printing operation. During print head return, the ribbon is alsodisplaced. Thus the ribbon must be accelerated up to a constant speed,advanced at that constant speed for a period during which the current ismonitored, decelerated and then positioned so as to minimize ribbonwastage. Driving a ribbon in this manner during intermittent printingoperations is a relatively simply matter as all that is necessary is toensure that the necessary motion of the ribbon incorporates a period ofconstant speed displacement during which current can be monitored. Incontinuous printing apparatus the problem is different as the ribbon ismoving at a rate related to the substrate speed. Ribbon speeds of lessthan 50 mm per second are difficult to utilize as there is a tendencyfor the ink to cool before it can be securely adhered to the substrate,and a wide range of substrate speeds above 50 mm per second must becatered for. Nevertheless, in order to save ribbon an amount of ribbonwill always be returned to the supply spool between successive printingoperations. It is necessary to ensure that the ribbon is returned in amanner such that the ribbon travels in the return direction for asufficient period of time at a constant velocity to enable an accuratemeasurement of motor currents. It may be that to achieve this it isnecessary for the ribbon to be “over-returned” so that before the nextprinting operation the ribbon has to be advanced to compensate for thisover-return. For both continuous and intermittent printing over-returnmay be used to ensure that sufficient ribbon is transported to providean accurate measurement during the tension measuring part of eachprinting cycle.

Preferably the motor currents are sampled over a period of timecorresponding to for example the travel of the ribbon through a distanceof at least 10 nun at a constant velocity. For example the current couldbe sampled at regular intervals with the interval between successivesamples corresponding to for example one quarter of a step of the motor.The samples are added together and the sum is divided by the number ofsamples taken. This gives an average current which is reasonablyrepresentative of the power being drawn by the associated stepper motor.

An analysis of the waveforms of current supplied to stepper motors inthe described embodiment shows that, in addition to the currentfluctuations resulting from the pulse width modulated nature of motorcontrol, there is a substantial amount of variation in the waveformswhich will mean that individual samples may not be representative of thepower being drawn by the motors. A more accurate representation of thatpower can be obtained if the monitored signals are passed through a lowpass filter (not shown) before being averaged.

FIG. 19 illustrates one approach to the monitoring of changing spooldiameters during ribbon usage. Alternative approaches are possiblehowever and one such alternative approach is described with reference toFIG. 20.

Referring to FIG. 20, Ar and A, are the areas of spools 7 and 11 (seeFIG. 1) respectively, d is the inner diameter of the spools and Dr andDs are the outer diameters of the spools at any given time. Hence:A _(r) +A _(s)=constant   (4)A _(r)=(D _(r)/2)²−(d/2)²   (5)A _(s)=(D _(x)/2)²−(d/2)²   (6)

Substituting from (5) and (6) into (4) gives:D _(r) +D _(s)=constant=D_(rc) +D _(sc)   (7)

Where D_(rc) and D_(sc) are rewind and supply spool diametersrespectively at initial calibration time.

-   Current diameter ratio R=D_(r)/D_(s)-   Therefore rearranging this D_(s)=D_(r)/R-   And also D_(r)=RD_(s)

Substituting in (7) gives: $\begin{matrix}{D_{r}^{2} = {D_{r}^{2}/R^{2}}} \\{= {D_{rc}^{2} + D_{sc}^{2}}} \\{= {R_{c}^{2} + D_{sc}^{2} + D_{sc}^{2}}} \\{= {D_{sc}^{2}\left( {R_{c}^{2} + 1} \right)}}\end{matrix}$

where R_(c) is the ratio of rewind to the supply reel diameter atinitial calibration.

Therefore D_(r) ²(R²+1)/R²=D_(sc) ²(R_(c) ²+1) and D_(r)²=[R²/(R²+1)][D_(sc) ²(R_(c) ²+1)].

So knowing the initial calibration spool diameters ratio (R_(c)), supplyspool diameters ratio (R_(c)), supply spool diameter at calibration(D_(sc)) and the current spool diameters ratio (R), the current diameterof either or both spools D_(r) or D_(s) can be derived.

In some applications it may be possible only to present a cassettecarrying a substantially empty take-up spool and a substantially fullsupply spool of known outside diameter. In such circumstances it wouldnot be necessary to determine the initial spool diameters. In generalhowever it is to be preferred to directly measure the spool diameters asit is likely that machine operators will at least on occasions usenon-standard spool configurations (for example ribbon which has beenpartially used on an earlier occasion).

As an alternative to the approach described above with reference to FIG.18 and equations 1 to 3, it is possible to derive an approximation ofribbon tension by relying upon the difference between the currents drawnby the two motors. This difference current is a function of themagnitude of the tension in the ribbon between the two motors and may beused as a control parameter such that for example, when the magnitude ofthe difference in current falls outside an acceptable tolerance band,the previously assumed ratio of the spool outside diameters is adjusted,resulting in a small change in the speed at which the two motors aredriven. This speed adjustment compensates for the updated spool diameterratio value. The “optimum” value of the difference current and itstolerance band will change as the spool diameters change. Theappropriate value for a particular set of circumstances may be foundfrom experimentation and stored in an optimum difference current profiletable which can be looked up as necessary.

No reference has been made in the above description to ribbon width,that is the dimension perpendicular to the direction of ribbon advance.It may be appropriate to provide a user with the option to manuallyenter a ribbon width value so as to enable the system to adjust thepredetermined tolerance limits and PID control gain constants referredto above to take account of tape-width dependent characteristics of theapparatus, e.g. to select different target limits for the measuredtension t (equation 3).

As discussed above, in transfer printers it is necessary to accuratelyposition the print head relative to the platen which supports thesubstrate to be printed if good quality print is to be produced,particularly at high printing speeds. The described exemplary embodimentavoids the need to make these mechanical adjustments to optimize printhead angle by making use of the fact that the print head is mounted on adisplaceable carriage.

FIG. 21 shows the roller platen 30, the print head edge 32 and the peeloff roller 33 as shown in FIG. 5. The line 109 represents the adjacentedge of the cover plate 21. The broken line 110 represents the positionof a tangent to the roller platen 30 at the point of closest approach ofthe print head edge 32 (it will be appreciated that during printing asubstrate and a print ribbon will be interposed between the edge 32 andthe roller platen 30). The line 111 represents a radius extending fromthe rotation axis 112 of the roller platen 30. The line 113 represents anotional line through the axis 112 parallel to the edge 109. The line113 represents no more than a datum direction through the axis 112 fromwhich the angular position of the radius 111 corresponding to angle 114can be measured.

Angle 115 is the angle of inclination of the print head relative to thetangent line 110. This angle is critical to the quality of printproduced and will typically be specified by the manufacturer as havingto be within 1 or 2 degrees of a nominal value such as 30 degrees.Different print heads exhibit different characteristics however and itis desirable to be able to make fine adjustments of say a degree or twoof the angle 115.

It will be appreciated that the angle 115 is dependent firstly upon thepositioning of the print head on its support structure and secondly bythe position of the tangent line 110. If the print head was to be movedto the right in FIG. 21, the angular position of the print head relativeto the rotation axis of the roller will change. That angular position isrepresented by the magnitude of the angle 114. As angle 114 increases,angle 115 decreases. Similarly, if the print head shown in FIG. 21 wasto be moved to the left, the angle 114 representing the angular positionof the print head relative to the rotation axis of the roller woulddecrease and the angle 115 would increase. This relationship makes itpossible for an installer to make adjustments to the print head anglesimply by adjusting the position adopted by the carriage 36 on the track22 (see FIG. 2) during printing. Thus an installer would initiallyposition the print head so that it would assume a nominal position inwhich the angle 114 would be approximately 90 degrees. A test print runwould then be used to assess print quality, the print head would bedisplaced relative to the track, a fresh print run would be conducted,and so on until the resultant print quality was optimized. There is norequirement for the installer to make mechanical adjustments to theposition of the print head on its support.

The printing methods described with reference to FIGS. 13 to 16 make itpossible to increase printing speed by reducing the distance that theprinter ribbon has to be advanced between successive printing phases insuccessive printing cycles. FIG. 22 illustrates the appearance of aprinted substrate at the left hand side, and the appearance of anassociated printer ribbon after first, second, third and fourth printingoperations respectively. It will be seen that alternate images are madeup of slightly offset printed lines, that offset making it possible forthe printer head to traverse the printer ribbon as described withreference to FIGS. 13 and 16 such that successive images are generatedin part from overlapping portions of the printer ribbon. The speed ofadvance of the printer ribbon for a given substrate speed and imagereproduction rate can be doubled. In this context, the term “printingcycle” is used to refer to a full cycle of activity which is performedin the interval between a printer head being first pressed into contactwith a printer ribbon so as to transfer ink from that ribbon to startthe formation of a first image until the print head is again broughtinto contact with the printer ribbon so as to initiate the transfer ofink which will form a second image. If the printing cycle relates to acontinuous printing machine, a full printing cycle includes an initialprinting phase in which the print head is stationary and the printerribbon is transported with the substrate to be printed past the printhead, and a subsequent non-printing phase during which the substratecontinues to be transported past the printing head, the print head isretracted from contact with the print ribbon, the direction of transportof the print ribbon is reversed, and then the print ribbon is again fedforward until it is traveling in the direction of the substrate,whereafter the printing phase of the next printing cycle is initiated.In an intermittent printer, the printing cycle is initiated with thesubstrate and ribbon stationary (unless the system is relying upon slipprinting), the print head is advanced across the ribbon and substrateduring a printing phase of the cycle, the print head is then retractedfrom the print tape and returned to its initial position, and thesubstrate and printer ribbon are advanced in readiness for theinitiation of the next print cycle.

Thus, during the printing phase of each printing cycle, the print headtraverses a predetermined length of ribbon either as a result ofdisplacement of the print head relative to a stationary or slower movingprint ribbon, or as a result of displacement of the print ribbonrelative to the print head. Thereafter the print ribbon is advanced apredetermined distance. The magnitude of that predetermined distance ofribbon advance is in many applications a limiting factor on the maximumspeed of the overall apparatus. In known printers the predetermineddistance of ribbon advance is generally at least as long as thepredetermined length of ribbon which is traversed by the print head. Thedescribed apparatus makes it possible to operate in a manner in whichthe predetermined distance of ribbon advance is less than thepredetermined length of ribbon traversed by the print head.

Referring to FIG. 22, the left hand side of the Figure shows foursuccessive images deposited on a substrate, each image being the same.The right hand section of FIG. 22 shows the original image which has tobe reproduced on the substrate. The four intervening sections illustratethe appearance of the print ribbon after the printing of the four imagesshown on the left hand side of FIG. 2. Assuming operation inintermittent printing mode, the substrate is advanced by an equaldistance between each of the successive printing cycles. The substrateis stationary during each printing cycle, as is the ribbon. Eachprinting cycle includes an initial printing phase during which the printhead is swept across the print ribbon so as to traverse a length of theribbon corresponding to the length of the image formed on the substrate,followed by a further phase in which the print head is returned to itsoriginal position and the ribbon is advanced a distance corresponding tohalf the length of the ribbon which is swept by the print head duringthe printing phase. During that first printing phase, only half of theprinting elements supported by the print head are energized, and thusthe image deposited on the substrate is in the form of a series ofparallel lines. During the next printing phase, the print head is againswept across the tape through a distance corresponding to the length ofthe image, but during that motion printing elements of the print headare energized which contact different parts of the tape from thosecontacted by energized printing elements during the first printingcycle. At the end of the second printing cycle, the print head is againreturned to its initial position and the ribbon is advanced by half thelength of the image formed on the substrate. Counting from the left inFIG. 22, the second, third, fourth and fifth sections of this Figureshow the appearance of the print ribbon after each of the first, second,third and fourth print cycles have been completed. It will be noted thatall of the images formed on the substrate are substantially the same,the only difference between successive images on the substrate beingthat one is made up of lines off set relative to lines forming theadjacent image.

The output represented in FIG. 22 is produced using a print head inwhich the print elements are arranged in a linear array with the oddnumbered printing elements in the array being allocated to one group andthe even numbered print elements in the array being allocated to theother group. This makes it possible to alternate between the groups sothat the distance advanced by the ribbon during each printing cycle isonly half of the length of ribbon from which ink is released during eachcycle. It will be appreciated that the printing elements could bearranged in three, four or more groups, the groups being energized in apredetermined cycle such that for example in the case of a three grouparrangement the distance advanced by the ribbon in each printing cyclecould be only one third of the length of printer ribbon swept by theprint head in any one cycle.

Although this aspect of the exemplary embodiment has been described indetail in the context of intermittent printing, it will be appreciatedthat the same technique could be applied to a continuous printingapparatus in which relative movement between the printing ribbon and theprint head is the result of transport of the ribbon past a stationaryhead rather than transport of a print head relative to a stationaryribbon.

1. A tape drive comprising: two motors, at least one of which is astepper motor; two tape spool supports on which spools of tape aremounted, each spool being drivable by a respective motor; a controllerfor controlling energization of the motors such that the tape istransported in at least one direction between spools mounted on thespool supports; wherein the controller is operative to energize bothmotors to drive the spools of tape in the direction of tape transport,and wherein the controller is operative to monitor tension in a tapebeing transported between spools mounted on the spool supports and tocontrol the motors to maintain the monitored tension betweenpredetermined limits.
 2. A tape drive according to claim 1, wherein thecontroller is arranged to control the motors to drive the spools totransport tape in both directions between the spools.
 3. A tape driveaccording to claim 1, wherein both of the motors are stepper motors. 4.A tape drive comprising: two motors, at least one of which is a steppermotor; two tape spool supports on which spools of tape are mounted, eachspool being drivable by a respective one of said motors; a controlleradapted to control energization of said two motors such that tape istransported in at least one direction between spools of tape mounted onthe spool supports; wherein the controller energizes both said motors soas to push-pull drive the spools in a tape transport direction; andwherein tension in driven tape disposed between the spools is controlledsolely by control of the two energized drive motors.
 5. A tape drivecomprising: two motors, at least one of which is a stepper motor; twotape spool supports on which spools of tape are mounted, each spoolbeing drivable by a respective one of said motors; a controller adaptedto control energization of said two motors such that tape is transportedin at least one direction between spools of tape mounted on the spoolsupports; and wherein the controller (a) energizes both said motors soas to push-pull drive the spools in a tape transport direction, (b)monitors tension in transported tape disposed between said spools and(c) controls said two motors to maintain the monitored tension betweenpredetermined limits.
 6. A tape drive as in claim 4 or 5, comprising: amonitor adapted to monitor electrical voltage and/or current supplied toat least one of motors and to calculate an estimate of tape tension fromthe monitored electrical voltage and/or current.
 7. A tape drive as inclaim 6 wherein said monitor is operative only when tape transport speedis substantially constant.
 8. A tape drive as in claim 4 or 5 whereinboth of said motors are stepper motors.
 9. A tape drive as in claim 8,wherein calibration data is recorded for each stepper motor, thecalibration data representing power consumption for the stepper motor ateach of a series of step rates under no tape load conditions, and ameasure of tape tension is calculated by reference to a measure of motorstep rate, the calibration data being related to the step rate, andpower consumed by the motor.
 10. A tape drive as in claim 9 furthercomprising: means for deriving a measure of the difference or ratiobetween the currents supplied to the two motors, and means forcontrolling stepping of the motors in dependence upon the difference orratio measure.
 11. A tape drive as in claim 4 or 5 further comprising:means for monitoring the outside diameters of the tape spools, and meansfor calculating tape tension by reference to the monitored diameters.12. A tape drive as in claim 5 wherein the means for monitoring theoutside diameter monitors the outside diameter of the spools for each ofa plurality of diameters which are mutually inclined to each other. 13.A tape drive as in claim 5 wherein: the controller maintains motor speedconstant during periods in which the difference or ratio measure iswithin each of a series of tolerance bands defined between upper andlower limits, and means are provided for adjusting the tolerance bandsin dependence upon the ratio of the outside diameters of the spools. 14.A method for operating a tape drive comprising two motors, at least oneof which is a stepper motor, and two tape spool supports on which spoolsof tape are mounted, each spool being drivable by a respective one ofsaid motors, said method comprising: controlling energization of saidtwo motors such that tape is transported in at least one directionbetween spools of tape mounted on the spool supports to push-pull drivethe spools in a tape transport direction, and controlling tension in thedriven tape between spools solely by control of the two energized drivemotors.
 15. A method for operating a tape drive comprising two motors,at least one of which is a stepper motor, and two tape spool supports onwhich spools of tape are mounted, each spool being drivable by arespective one of said motors, said method comprising: controllingenergization of said two motors such that tape is transported in atleast one direction between spools of tape mounted on the spoolsupports, to push-pull drive the spools of tape in a tape transportdirection; monitoring tension in transported tape between said spools;and controlling said two motors to maintain the monitored tensionbetween predetermined limits.
 16. A method as in claim 14 or 15 furthercomprising: monitoring electrical voltage and/or current supplied to atleast one of said motors and calculating an estimate of tape tensionfrom the monitored electrical voltage and/or current.
 17. A method as inclaim 16 wherein said monitoring is done only when tape transport speedis substantially constant.
 18. A method as in claim 14 or 15 wherein:said controlling step calculates a length of tape to be added to orsubtracted from tape extending between said spools in order to maintaintension in said tape between upper and lower limit values then controlssaid motors to add or subtract the calculated length of tape to the tapeextending between said spools.
 19. A method as in claim 14 or 15 whereinboth of said motors are stepper motors and the motors are controllableto transport tape in either direction between the spools.
 20. A methodas in claim 19, wherein calibration data is recorded for each steppermotor, the calibration data representing power consumption for thestepper motor at each of a series of step rates under no tape loadconditions, and a measure of tape tension is calculated by reference toa measure of motor step rate, the calibration data being related to thestep rate, and power consumed by the motor.
 21. A method as in claim 20further comprising: deriving a measure of the difference or ratiobetween the currents supplied to the two motors, and controllingstepping of the motors in dependence upon the difference or ratiomeasure.
 22. A method as in claim 15 wherein: a control algorithmoperates cyclically such that during one cycle the length of tape to beadded or subtracted is calculated and during a subsequent cycle themotors are controlled to adjust the length of tape disposed between thespools.
 23. A method as in claim 21 wherein: motor speed is maintainedconstant during periods in which the difference or ratio measure iswithin each of a series of tolerance bands defined between upper andlower limits, and the tolerance bands are adjusted in dependence uponthe ratio of the outside diameters of the spools.
 24. A method as inclaim 14 or 15 further comprising: monitoring the outside diameters ofthe tape spools, and calculating tape tension by reference to themonitored diameters.
 25. A method as in claim 24 wherein the outsidediameter of the spools is monitored for each of a plurality of diameterswhich are mutually inclined to each other.
 26. A tape drive comprising:two stepper motors; two tape spool supports on which spools of tape aremounted, each spool being drivable by a respective one of said steppermotors; a step motor controller controlling energization of said twostepper motors such that tape can be transported in both directionsbetween spools of tape mounted on the spool supports; wherein thecontroller drives each of said motors with drive pulses so as topush-pull drive the spools in each tape transport direction; and whereintension in driven tape disposed between the spools is controlled solelyby changing drive pulses supplied by the controller to the steppermotors.
 27. A tape drive comprising: two motors, at least one of whichis a stepper motor; two tape spool supports on which spools of tape aremounted, each spool being drivable by a respective motor; a controllerfor controlling energization of the motors such that the tape istransported in at least one direction between spools mounted on thespool supports; wherein the controller is operative to energize bothmotors to drive the spools of tape in the direction of tape transport,and wherein tension in driven tape disposed between the spools is setsolely by control of the two energized drive motors.
 28. A tape drivecomprising: two stepper motors; two tape spool supports each having amount for a spool of tape, each spool support being drivable by arespective one of said stepper motors; a stepper motor controllercontrolling simultaneously energization of said two stepper motors todrive both of said spool supports and transport tape in both directionsbetween the supports; wherein the controller drives each of said motorswith drive pulses to push-pull drive the spool supports in each tapetransport direction; and wherein tension in tape disposed is controlledsolely by changing drive pulses supplied by the controller to thestepper motors.
 29. A tape drive comprising: two motors, at least one ofwhich is a stepper motor; two tape spool supports, wherein each spoolsupport is driven by a respective one of the motors; a controller forcontrolling energization of the motors such that tape is transported inat least one direction between the spool supports; wherein thecontroller is programmed to simultaneously energize the two motors todrive both supports and transport tape between the supports, and whereinthe controller is programmed to control tension of the tape by thecontrolled energization of the two motors.
 30. A method for operating atape drive comprising two motors, at least one of which is a steppermotor, and two tape spool supports, each spool support being drivable bya respective one of said motors, said method comprising: controllingsimultaneous energization of the two motors such that tape istransported in at least one direction between the spools supports topush-pull drive the spools in a tape transport direction, andcontrolling tension in the driven tape by controlling of theenergization of at least one of the drive motors.
 31. A method foroperating a tape drive comprising two motors, at least one of which is astepper motor, and two supports which each receive a spool of tape withtape extending between the spools, each spool support being drivable bya respective one of said motors, said method comprising: controllingsimultaneous energization of the two motors such that tape istransported in at least one direction between the spool supports topush-pull drive tape in a tape transport direction; monitoring tensionin the transported tape between said spools; and controlling said twomotors to maintain the monitored tension between predetermined limits.